1. Field of the Invention
The present invention relates to a modulation circuit, a modulation method, a program, and a communication apparatus.
2. Description of the Related Art
In recent years, various kinds of radio communication apparatus such as a mobile phone, a cordless phone, an apparatus including a WLAN (Wireless Local Area Network) and an apparatus including a WUSB (Wireless Universal Serial Bus) have been widely used. The above radio communication apparatuses have, for example, an orthogonal modulator to allow a digital signal as a transmission signal to be transmitted to external devices after being modulated to a signal of a desired frequency (for example, a RF (Radio Frequency) signal). The orthogonal modulator has, for example, a DAC (Digital to Analog Converter) to modulate a digital signal into an RF signal. Here, a DAC output spectrum may have spectrum folding of the current signal generated by in higher harmonics of sampling frequencies. Thus, the orthogonal modulator has a filter to suppress a folded spectrum. The above filter includes, for example, a reconstruction filter (also referred to as an anti-imaging filter).
Among these, a technologies to implement a filter using a digital circuit are being developed. Non-patent Document 1 is exemplified as technology to implement a reconstruction filter using a digital filter by sampling a digital signal at a frequency twice the sampling frequency or more.
[Non-patent Document 1] Petri Eloranta, Pauli Seppinen, Sami Kallioinen, Tuomas Saarela, Aarno Parssinen, “A Multi mode Transmitter in 0.13 um CMOS Using Direct-Digital RF Modulator”, IEEE JOURNAL OF SOLID-STATE CIRCUIT, VOL. 42, NO. 12, DECEMBER 2007.